Crude oils are distilled in oil refinery equipment to produce various fractions such as a gasoline fraction, a fuel oil fraction, or a lubricating oil fraction, and others. Corrosion problems are often encountered in the fractionation of crude oil which is carried out in a crude column. Corrosion problems in a crude unit can be due to any one of or a combination of components found in crude oil, chemicals used in the distilling process, and environmental conditions. The present invention is directed to corrosion problems which are due to added chemicals and constituents formed during the actual distillation process. The main impurities in crude oils which contribute to corrosion of the overhead condensing system of distillation units and other structures of the refinery equipment include chloride and sulfur-containing salts, naphthenic and other organic acids and inorganic acids. It is an object of this invention to provide a method for abating corrosion in an atmospheric column overhead system operated at temperatures above the nominal aqueous dew point where unusually severe corrosion is caused by ammonium salts of sulfuric acid.
Acid-forming substances such as sulfur oxides cause severe corrosion of carbon steel from which conventional petroleum refining equipment is constructed. While it would be possible to fabricate refinery equipment from alloys which are less prone to corrosive attack, the cost of such equipment would be inordinately high and would make any process being conducted with such equipment uneconomical.
It is therefore essential to control corrosion and make possible the use of carbon steel as construction material in the overhead condensing systems.
It has now been found that the corrosion in the overhead of a crude column caused by high levels of ammonium bisulfate is abated by maintaining a level of chloride in the feed stream such that the molar ratio of ammonium chloride to ammonium disulfate in the crude column overhead transfer line is greater than about 20.